Dental restorations using nanocrystalline materials

ABSTRACT

Dental articles are produced using relatively low sintering temperatures to achieve high density dental articles exhibiting strengths equal to and greater than about 700 MPa. Ceramic powders comprised of nanoparticulate crystallites are used to manufacture dental articles. The ceramic powders may include sintering agents, binders and other similar additives to aid in the processing of the ceramic powder into a dental article. The ceramic powders may be processed into dental articles using various methods including, but not limited to, injection molding, gel-casting, slip casting, or electroforming, hand, cad/camming and other various rapid prototyping methods. The ceramic powder may be formed into a suspension, pellet, feedstock material or a pre-sintered blank prior to forming into the dental article.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of and claims priority toU.S. application Ser. No. 12/490,539, filed Jun. 24, 2009, now U.S. Pat.No. 7,806,694, entitled Methods of Manufacturing Dental RestorationsUsing Nanocrystalline Materials, which is a divisional application ofand claims priority to U.S. application Ser. No. 10/857,482, filed May28, 2004, now U.S. Pat. No. 7,655,586, entitled Dental RestorationsUsing Nanocrystalline Materials and Methods of Manufacture, which claimspriority to U.S. application Ser. No. 60/474,166 filed May 29, 2003,entitled Methods of Fabricating Dental Restorations UsingNanocrystalline Materials, all of which are hereby incorporated byreference.

1. Field of the Invention

This invention relates to nanocrystalline ceramic powders especiallyuseful for fabricating dental restorations and methods of fabricatingdental restorations using ceramic powders comprising nanoparticles.

2. Background of the Invention

Techniques of fabricating all-ceramic dental restorations by hand andmethods using commercial high-tech systems such as CAD/CAM systems eachhave their limitations and target different segments of the dentallaboratory market. There are two main challenges restricting widespreaduse of high-strength ceramic materials for cost-effective fabrication ofdental restorations and both challenges are related to the sinteringstep of the operation. High-strength ceramic materials are crystallinematerials formed from powder and require high temperatures for sinteringthat result in substantial shrinkage. Any technique enabling use ofthese materials for dental restorations should offer ways to (1)compensate for shrinkage and (2) provide a furnace capable of reachingthe temperatures necessary to sinter the material to nearly fulldensity.

A technique reportedly providing the highest strength for manuallyproduced copings, the Vita® In-Ceram™ method (developed by VITAZahnfabrik), has been advertised as yielding a material with flexuralstrength of about 500 MPa or even higher. This technique has not becomepopular mostly due to esthetic limitations and a tedious multi-stepfabrication procedure that includes a glass infiltration step. Thisglass infiltration technique is one way to circumvent theabove-mentioned challenges. Vita® In-Ceram™ copings are slip-cast on agypsum die and soft-sintered with negligible shrinkage. The final glassinfiltration step does not require a special furnace. The resultingproduct is a fully dense restoration having undergone no shrinkage.Nonetheless, the presence of a glass phase in the glass-infiltratedceramics makes it inferior to corresponding crystalline ceramics inmechanical strength and chemical durability.

Currently available CAD/CAM systems are capable of compensating forshrinkage by milling enlarged shapes. Moreover, high-temperatureisotropic sintering results in fully dense and accurate final shapes.However, CAD/CAM systems and procedures are expensive and not affordableby small labs. For example, two of the most recently developedcommercial state-of-the art CAD/CAM systems, the LAVA™ system (availablefrom 3M ESPE) and the CERCON® system (available from Dentsply/Degussa),require the purchase of a scanner, milling machine and high-temperaturesintering furnace and are currently priced in the range of approximately$60,000-$180,000. Both of the aforementioned CAD/CAM systems employsoft-sintered zirconia blocks. The blocks are milled to enlarged shapesand subsequently sintered to full density. Both systems are advertisedas yielding materials having a flexural strength of about 900 MPa orhigher.

Glass-ceramic materials obviate the need to compensate for shrinkage andhigh temperature sintering. They can be hand-built on a refractory dieand sintered at fairly low temperatures to assure accuracy of the finalshape. One example of such a material is an OPC™ Low Wear (availablefrom Pentron Laboratory Technologies, LLC) porcelain jacket crown (PJC).Glass-ceramic materials can also be injection molded into a refractoryinvestment mold formed by the lost wax technique. Examples ofcommercially available materials used in this process include OPC®porcelain, and OPC® 3G™ porcelain, IPS Empress® porcelain and Empress 2™porcelain. The physical mechanism underlying the highprocessability/formability of these glass-ceramics is the viscous flowof its glass component. The glass-ceramic materials listed above(Optec™, OPC® and OPC® 3G™, Empress® and Empress2™ materials) have fromabout 40% to about 60% of a glass phase which serves as a matrix inwhich from about 40% (e.g., Optec) to about 60% of crystals (e.g.,Empress2) are embedded. These crystals are grown in-situ bycrystallization heat-treatment of the parent glass. Alternatively, in amethod described by Hoffman in U.S. Pat. Nos. 5,916,498, 5,849,068 and6,126,732, in order to improve processability of the material, up to 50%glass is added to the crystalline ceramic powder. As a result, thereported flexure strength is limited to less than 600 MPa. Byintroducing a glass phase into the microstructure, strength iscompromised to gain better processability.

Sintering of glass-ceramic powders is a relatively fast process comparedto sintering of crystalline ceramic powders due to the viscous flowmechanism of the former, which is associated with higher densificationrates, but the presence of the residual glass phase limits the strengthof the final product. Another benefit of the viscous flow mechanism isthat the glass ceramic conforms to the shape of the die during sinteringwithout cracking. On the other hand, crystalline ceramics can be muchstronger than glass ceramics, but crystalline ceramics sinter by asolid-state diffusion mechanism that is intrinsically slow creatinginhomogeneous shrinkage, generating significant sintering stresses thatmay result in associated cracking. Liquid phase sintering induced by theaddition of sintering aids greatly enhances sinterability of crystallineceramics by promoting particle rearrangement and solution-precipitationmechanisms but such mechanisms do not achieve all the advantages of theviscous flow mechanism.

At the same time many experimental and theoretical studies reveal adecrease of the melting temperature of nanometallic particles incomparison with the melting temperature of conventional bulk metals. Itsmagnitude depends mostly on particle size and crystal structure as wellas particle surface conditions and the host matrix environment such asthe presence of impurities, level of agglomeration, coating, depositionsubstrate and the like. Usually, melting is associated with apre-melting process resulting in a change in shape of the nanoparticlesfollowed by the formation of a liquid skin on the melting nanoparticles.The liquid skin thickness increases during melting gradually consumingthe solid particle core. Transmission electron microscopy studies, suchas the one discussed in “Shape Transformation and Surface Melting ofCubic and Tetrahedral Platinum Nanocrystals” by Z. L. Wang, J. M.Petroski, T. C. Green and M. A. El-Sayed, J. Phys. Chem. 102, (32)6145-6151 (1998), have established that 8 nanometer platinumnanoparticles begin to melt at about 600° to about 650° C., which is amuch lower temperature than the melting point of bulk platinum at 1769°C. At about 500° C., cubic particles change their shape to a sphericalshape with surface melting occurring at about 600° C. to about 650° C.The molten layer surrounding solid cores of platinum nanocrystals isabout 1 nm in thickness at 600° C. and the thickness increases withtemperature as the nanoparticles continue to melt. The “melting pointdepression” abbreviated as MPD is a thermodynamically driven phenomenonand can be explained by a drastic increase in the surface area/volumeratio in nano-particulate materials and the corresponding increase intheir specific surface energy. This leads to a size-related dependenceof melting temperature that is roughly close to 1/d functionality, whered is the mean particle size, and contains surface tension coefficients,latent heat of melting and the molten skin thickness as parameters.

Table 1 presents some experimental data illustrating the difference inmelting temperatures for nanoparticles and the corresponding bulk metalsand semiconductors.

TABLE 1 Melting temperatures of the selected nanomaterials Nano-Material Bulk Melting Nano- Nano- Melting Melting Point ParticleParticle Temperature Temperature Depression Melting/Surface MaterialShape size, nm ° C. ° C. ° C. T_(Mnano)/T_(Mbulk) Melting Ref. Pt cubic8 650 1769 1100 0.37 surface melting [1] Au spherical 4 650 1057 4000.61 melting [2] Ag spherical 7 470 961 490 0.49 melting [2] Pd wireDiameter 4.6 300 1445 1100 0.21 melting [3] Length 202 Sn spherical 10 153 232 80 0.66 melting [4] CdS spherical 4 630 1678 1080 0.38 melting[5] Ge wire Diameter 55 650 930 280 0.70 surface melting [6] Length 1000from ends [1] “Shape Transformation and Surface Melting of Cubic andTetrahedral Platinum Nanocrystals,” Z. L. Wang, J. M. Petroski, T. C.Green and M. A. El-Sayed, J. Phys. Chem. 102, (32), 6145-6151 (1998).[2] “Size-Dependent Melting Temperature of Individual Nanometer-SizedMetallic Clusters,” T. Castro, R. Reifenberger, E. Choi and R. P.Andres, Phys. Rev., B 42 (13), 8548-8556 (1990). [3] “Size ControlledSynthesis of Pd Nanowires Using a Mesoporous Silica Template ViaChemical Vapor Infiltration,” K-B Lee, S-M Lee, and J. Cheou, Adv.Mate., 13 (7), 517-520, (2001). [4] “Size-Dependent Melting Propertiesof Small Tin Particles: Nanocalorimetric Measurement,” S. L. Lai, J. Y.Guo, V. Petrova, G. Ramanath and L. H. Allen, Phys. Rev. Lett., 77(1),99-102, (1996). [5] “Melting in Semiconductor Nanocrystals,” A. N.Goldstein, C. M. Echer and A. P. Alivisatos, Science, 256, 1425-1427,(1992). [6] “Melting and Welding Semiconductor Nanowires in Nanotubes,”Y. Wu and P. Yang, Adv. Mater., 13 (7), 520-523, (2001).

Onset of surface melting occurs usually at temperatures even lower thanthe temperature at which the entire nanoparticle melts. It can bespeculated that the “molten shells” of the pre-melted nanoparticles workas “a lubricant” inducing higher mobility of the particles and higherdiffusion rates and hence facilitating densification at temperaturesmuch lower than 0.6 of the melting point (T_(m)).

It can be further speculated that thermodynamic considerationsexplaining the mechanism of MPD described above should hold for ceramicnanoparticles as well. Nevertheless, the MPD effect is not very wellstudied in ceramics for obvious reasons—even the depressed melting pointanticipated for ceramic nanoparticles will still be very high making itextremely difficult to conduct observations similar to those for metalsand semiconductors described above in Table 1.

For example, the melting point (T_(M)) for pure alumina and zirconia are2050° C. and 2700° C., respectively, and therefore the MPD effect of theorder of 0.5T_(M) will result in melting temperatures for nano-aluminaand nano-zirconia particles of about 1025° C. and 1350° C. However,there are some indirect indications that MPD does occur in nanoceramicssuch as extremely low sintering temperatures for nanopowders as reportedin R. A. Kimel, Aqueous Synthesis and Processing of Nanosized YttriaTetragonally Stabilized Zirconia, Ph.D. Thesis, The Pennsylvania StateUniversity, the Graduate School, the College of Earth and MineralSciences, (2002) and in G. Skandan, H. Hahn, M. Roddy and W. R. Cannon,“Ultrafine-Grained Dense Monoclinic and Tetragonal Zirconia,” J. Am.Ceram. Soc., vol. 77, no. 7, pp. 1706-10 (1994), which are both herebyincorporated by reference.

These studies reported onset of densification at surprisingly lowtemperatures of about 0.3T_(M) as well as a surprising and uniqueability of nanoceramics to be translucent at fairly high levels ofporosity.

Some studies reported extreme difficulty in sintering nanoceramics tofull density due to rapid grain growth. For example, Skandan et al.(cited above) observed that grains grew 15 times the initial particlesize in the case of nano-zirconia. The other major obstacle encounteredwith the use of nanoparticles in the fabrication of dental articles isrelated to difficulties in the consolidating of bulk shapes usingconventional methods like powder compaction and slip-casting. It thescope of the present invention to utilize the advantages ofnanoparticulate ceramics while successfully overcoming the obstaclescurrently hampering use of such nanoparticulates as dental ceramics.

It is desirable to provide dental ceramics having low sinteringtemperatures and high strengths. It would be beneficial to providedental ceramics having sintering temperatures that are low enough to besintered in existing dental furnaces, yet maintaining high strength andtranslucency. It is most desirable to provide processing techniques fordental ceramics that result in fully densified dental ceramics.

SUMMARY OF THE INVENTION

These and other objects and advantages are accomplished by the ceramicpowders of the present invention which are manufactured into dentalarticles. The ceramic powders may include sintering agents, binders andother similar additives to aid in the processing of the ceramic powderinto a dental article. The ceramic powders are comprised ofnanoparticulate crystallites. The ceramic powders may be processed intothe dental article using various methods including, but not limited to,injection molding, gel-casting, slip casting, or electroforming, handforming, cad/camming and other various rapid prototyping methods. Theceramic powder may be formed into a suspension, pellet, feedstockmaterial or a pre-sintered blank prior to forming into the dentalarticle.

Dental articles are produced using relatively low sintering temperaturesto achieve high density dental articles exhibiting strengths equal toand greater than about 700 MPa.

BRIEF DESCRIPTION OF THE DRAWINGS

Features of the present invention are disclosed in the accompanyingdrawings, wherein similar reference characters denote similar elementsthroughout the several views, and wherein:

FIG. 1 is a schematic diagram showing the structure of particlesdescribed herein;

FIG. 2 is a schematic diagram showing particle size distribution ofceramic powders;

FIG. 3 is a graph showing density versus sintering temperature ofnano-sized and conventional three mole percent yttria-stabilizedzirconia;

FIG. 4 is a schematic diagram of sample geometry and an orthogonalcoordinate system used for sintering shrinkage measurements;

FIG. 5 is a graph showing shrinkage of outside diameter L_(x) versusshrinkage of the height of cups made out of conventional zirconia powderand nanosized zirconia powder (NP-2); and

FIG. 6 is a graph showing shrinkage of outside diameter L_(y) versusshrinkage of the height of cups made out of conventional zirconia powderand nanosized zirconia powder (NP-2).

DETAILED DESCRIPTION OF THE INVENTION

This invention provides particulate materials that can be processed intodental restorations using both the most sophisticated state-of-the arttechnologies such as solid free form manufacturing (SFF) methods as setforth in U.S. Pat. No. 6,322,728, and copending commonly owned U.S.patent application Ser. Nos. 09/972,351 (US 2002/00335458), now U.S.Pat. No. 6,994,549, Ser. No. 10/053,430 (US 2002/0125592), now U.S. Pat.No. 6,808,659, and Ser. No. 09/946,413 (US 2002/0064745), now U.S. Pat.No. 6,821,462, all of which are hereby incorporated by reference, aswell as manual techniques similar to classic methods of buildingporcelain jacket crowns on a refractory die (e.g. OPC™ Low Wearporcelain jacket crowns made from powder or jacket crowns made usingtape-cast ceramic sheets as described in U.S. Pat. No. 5,975,905, whichis hereby incorporated by reference) or slip casting a ceramic slip ontoa porous die/mold.

The ceramic particulate materials of the present invention have complexhierarchical architecture with three levels of structural organization:nano-, micro-, and macro-level as shown in FIG. 1. On the nano-level(≦20 nm), the structure is based on nano-crystallites depicted at 10 aselemental building blocks. On the micro-level (0.1-20 microns), thestructure is formed from polycrystalline particles or agglomeratescomprised of clusters of nanocrystallites depicted at 12. On amacro-level (20-200 microns), polycrystalline particles are agglomeratedinto granules depicted at 14. These granules 14 are made by spray-dryingor fluidized bed agglomerating polycrystalline particles comprised ofnanoparticles of one or more kinds of materials including, but notlimited to, metallic and ceramic materials which may be fully orpartially calcined or still in the form of organic/inorganic precursors.Sintering aids are depicted interstially at 16. The advantage of usingnanoparticles is their drastically different sintering behaviorassociated with MPD. In much of the scientific literature, such as in,“Melting of Isolated Tin Nanoparticles” by T. Bachels, H-J Guntherodt,and R. Schafer, Phy. Rev. Lett., 85, (6), 1250-1253, (2000), which ishereby incorporated by reference, this effect is also referred to as“size dependence of melting temperature” in nano-materials. As a resultof this mechanism, sintering of nanoparticles is speculated to be aidedby the occurrence of surface pre-melting and hence, controlled bycapillary forces. Beneficial utilization of capillary forces through thehierarchical architecture of ceramic powders comprising nanoparticles isan essential feature of this invention. The hierarchical architecture ofnanocrystalline powders of this invention is specifically engineeredto 1) aid consolidation of particulates into green shape; 2) takeadvantage of capillary effects during sintering (i.e. liquid phasesintering and surface melting of nanoparticles) to maximize particlerearrangement, enhance sintering kinetics and lower the sinteringtemperature; and 3) control the size, distribution and morphology of theresidual porosity.

Examples of metallic powders useful herein include, but are not limitedto Si, Al, Mg, Zr, Y, Ce, Ta and mixtures thereof. These metals areprimarily chosen because they oxidize easily and form glass-formingoxides SiO₂, Al₂O₃, MgO, ZrO₂, Y₂O₃, CeO₂ and Ta₂O₅ that facilitateliquid-phase formation during sintering. Most of these oxides arecurrently used as sintering aids or dopants in the manufacture ofhigh-performance ceramics such as alumina, zirconia, silicon nitride andSIALON ceramics. The advantage of adding these elements in metallicrather than oxide form is that as nanophase metal particles they areextremely reactive, have high enthalpy of oxidation, i.e., generatehighly localized heat upon oxidation, and provide good coupling formicrowave energy.

Examples of ceramic nanocrystalline powders useful herein include, butare not limited to, oxide ceramics such as various forms andmodifications of zirconia, alumina, titania, silica, magnesia, yttria,ceria and solid solutions or mixtures thereof.

The metallic powders and ceramic nanocrystalline powders of the presentinvention have sintering temperatures lower than about 1300° C. andpreferably lower than about 1200° C. and most preferably not exceedingabout 1100° C. Sintering temperatures of lower than 1300° C. are themost economical since sintering can be carried out in the most commonresistance-heated furnaces having metallic heating elements. Each dentallab has at least one burn-out furnace with a maximum continuousoperating temperature of at least 1100° C. and a porcelain furnace witha maximum operating temperature of about 1200° C. The essential featureof this invention is that these powders can be processed into dentalrestorations using both the most sophisticated state-of-the-arttechnologies such as Solid Free Form Manufacturing (SFF) methods, alsoknown as Rapid Prototyping (RP), as well as manual techniques similar toclassic methods of building porcelain jacket crowns on a refractory die,or jacket crowns made using tape-cast sheets or slip casting a ceramicslip on a porous mold. Examples of SFF/RP methods includestereolithography (SLA) and photo-stereo-lithography including DigitalLight Processing (DLP) and Rapid Micro Product Development (RMPD) masktechnique, selective laser sintering (SLS), ballistic particlemanufacturing (BPM), fusion deposition modeling (FDM), multi-jetmodeling (MJM) and three-dimensional printing (3DP).

Particle size distribution of these powders made of granules andpolycrystalline particles is designed to improve handling for makingrestorations by hand or to optimize powder bed or feed-stockcharacteristics for specific SFF methods used. Engineered particulatematerials of the present invention have complex hierarchicalarchitecture with three levels of structural organization: nano-,micro-, and macro-level as shown in FIG. 1. On the macro-level theparticulates are formed into a nearly spherical shape with a diameterfrom about 10 microns to about 500 microns, more preferably from about20 to about 200 microns. These spherical particles should preferably besolid, not hollow. The size distribution of powders useful herein shouldbe optimized for the specific forming technique. For example, forbuilding by hand, the particle size distribution should be bimodal withthe fraction of finer particles fitting in the interstitials between thefraction of coarser particles. The ratio of the mean diameter of coarserparticles (D_(c)) to the mean diameter of finer particles (D_(f)),D_(c)/D_(f), should be more than 3 and preferably more than 6, whereasthe relative amount of finer particles is from about 10 wt % to about 25wt %. An example of particle size distribution is shown in FIG. 2. Curve20 depicts particle size distribution of nanophase powder facilitatingmanual build-up and curve 22 depicts typical volume based distributionfor dental porcelain with good “handling.”

In addition to using nanocrystalline powders to induce low sinteringtemperatures, sintering aids and binders/additives may be mixed with thenanocrystalline powders to further facilitate the action of capillaryforces to aid in powder consolidation and sintering. Binders used hereinmay include any known binder used in conventional powder processingmethods and may be compounds or mixtures of compounds activated by heat,light, or other types of radiation or by chemical reaction. Examples ofbinders/additives include, but are not limited to, polymeric binders,plasticizers, surfactants and dispersants such as polyacrylic binders,polyvinyl alcohol (PVA) binders, polyvinyl butyral binders, stearic andoleic acids, silanes, and various natural and synthetic waxes such asparaffin wax, polyethylene wax, carnauba wax, and bee's wax.

In accordance with a method herein, ceramic powder comprisingnanocrystals is mixed with a metallic sintering aid comprising metallicmicro- and/or macro-size particles. Other sintering aids and binders mayalso be added to the mixture. During mixing, the nanocrystalline powdersbecome coated with the additives using any of the availablecoating/agglomeration techniques including, but not limited to, spraydrying, fluidized bed agglomeration methods, dry and wet milling andmechanical alloying. In one of the preferred embodiments of the presentinvention, the additives comprise metallic particulates. In anotherembodiment, the additives comprise ceramic nanophases and/ornanocrystallites such as grain growth inhibitors. These sinteringaids/additives facilitate the thermal sintering/densification process.

After mixing, the mixture is then sintered at a temperature of less thanabout 1300° C., preferably at a temperature of less than about 1200° C.and most preferably in the range of temperatures from about 800° C. toabout 1150° C.

In yet another embodiment, microwave processing is used to densify theparticulate by sintering through the absorption of microwave energy.

Processes occurring during the melting of a material comprisingnanoparticles as described above are somewhat similar to the processesoccurring during liquid phase sintering as described in Fundamentals ofCeramics, M. Barsoum, McGraw Hill (1997). In both cases, the definingfactor is the presence of liquid and therefore the entire process iscontrolled by capillary forces. In contrast however, during liquid phasesintering the liquid phase is formed due to the addition of sinteringaids, and during the sintering of nanoparticles, formation of the liquidskin on nanoparticles occurs by the mechanism of surface melting,intrinsic to nanophase materials. This invention takes advantage of bothintrinsic liquid formation (due to surface melting of nanoparticles) andextrinisic liquid formation (due to sintering aids). The nanocrystallinestructure of the polycrystalline particles are combined with sinteringaids when they are agglomerated into granules. The granules themselvesare coated with a coating comprising sintering aids and agents to aidcapillary forces during sintering as well as in the forming of theshape. For example, handling of the powder/liquid paste-like mixture formanual wet-condensing of ceramic powder on a refractory die is primarilycontrolled by capillary forces.

Particulate material comprised of nanoparticles may behave in wayssimilar to glass-ceramic material due to a very significant fraction ofrelatively disordered material on grain boundaries. In addition,extremely high specific surface energy associated with nanoparticlesgreatly increases the driving force for densification. A high fractionof grain boundaries substantially alters sintering behavior ofnano-sized ceramics compared to that of conventional micron-sizedceramics. Surface melting of nanoparticles, resulting in liquid skinformation around nanoparticles, induces and promotes the mechanisms ofsintering previously associated with liquid phase sintering such asparticle rearrangement and solution-precipitation. At the same time,grain-boundary diffusion sintering mechanisms are greatly enhanced dueto the enormous surface area of the nanoparticles. Normally, thepresence of agglomerates inhibits densification during solid-statesintering, however, with added mechanisms of liquid phase sintering andsurface melting of nanoparticles, deliberate granulation of powder is anessential feature of this invention that facilitates beneficialcapillary effects during green shape fabrication and sintering.

It is expected that hand-built restorations will have some residualporosity after final sintering, however, the architecture of the powderis designed to minimize this residual porosity and spatially coordinateit to minimize its adverse effect on mechanical properties. It is nowrecognized that porosity is practically an unavoidable element ofmicrostructure and porous ceramics are not necessarily weak, as statedin “Fracture Energy of an Aligned Porous Silicon Nitride,” by Y.Inagaki, T. Ohji, S. Kanzaki and Y. Shigekaki, J. Am. Ceram. Soc., 83(7), 1807-1809, (2000). Porosity as an engineered element of themicrostructure of the materials of the present invention can becontrolled and spatially organized through engineered hierarchy of thestarting particulate material. It is well known in the art that thecritical flaw size causing brittle fracture of ceramics often scaleswith the particle size of the starting powder. In materials of thepresent invention, the powder is preferably spherical in shape promotingbetter flowability of the powder. In the powder herein, the pore sizescales with the diameter of interstitial sites formed between theparticles of the powder providing that the powder was carefullycondensed or compacted and attained the maximum green density of thecompact. The pore size and spatial distribution will be defined by thesize and spatial distribution of interstitials between the particles.For example, for a spherical powder with particle size distributionshown in curve 1 of FIG. 2 the largest pore diameter will be defined bythe size of the largest interstitial between the smallest sphericalparticles, which in this case is about 20 μm. The largest, octahedralinterstitial in close packed arrangement of 20 μm spheres will be(√3−1)μm×20=0.732×20 μm=14.64 μm.

The equation σ=K_(IC)/(Ya^(1/2)) calculates the strength based on thevalue of fracture toughness and the critical flaw size, where

-   -   K_(IC) is the fracture toughness;    -   Y is the geometric factor;    -   σ is the fracture strength;    -   √ is the square root; and    -   2a is the equivalent crack length associated with the critical        flaw.

For a yttria-stabilized tetragonal zirconia polycrystals (YTZP) materialwith K_(IC) of 6-9 MPa·m^(1/2), and a geometric factor (Y) of 2,strength will most likely be well in excess of 700 MPa.

TABLE 2 Largest Pore K_(IC), MPa · m^(1/2) Size, μm Y* (1.84-2.46)Strength, MPa 6 15 2 775 9 15 2 1162

Nanocrystalline particulate ceramic materials of this invention aresupplied as free-flowing powder, pre-sintered blanks, feed-stock (forinjection molding) and suspensions for slip-casting or electroformingand using fabrication techniques described below to provide materialswith flexure strength of at least 700 MPa and exceeding 1 GPa which ismore than enough even for multi-unit posterior restorations andcantilever bridges.

The following examples illustrate the invention.

EXAMPLE 1

Commercially available TZ-3Y-E (which is a yttria-stabilized tetragonalzirconia powder) powder manufactured by TOSOH Corporation (Japan) with acrystallite size of about 30 nm can be sintered to full density attemperatures as low as 1350° C., which is 150° C. lower than thesintering temperature for conventional yttria-stabilized tetragonalzirconia polycrystals (YTZP) powder. Onset of sintering normally occursat about 0.6 of the melting point (Tm) and the MPD effect describedabove results in a corresponding decrease in sintering temperature fornanopowder compared to conventional micron-size ceramic. If the size ofthe crystallites is reduced three times to about 10 nm, the anticipatedreduction in the sintering temperature will be about 450° C., i.e., YTZPpowder comprised of 10 nm nanoparticles is sinterable at about 1050° C.Thus, this powder may be sintered in a regular burn-out furnace with themaximum operating temperature of 1100° C. Example 2 below furtherillustrates the viability of sintering nanosized zirconia powders tonearly full density at temperatures lower than 1300° C. and preferablylower than 1100° C.

EXAMPLE 2 Low Temperature Sinterability of Nano-Zirconia Powders

Two commercially available nano-sized 3 mol % yttria stabilized zirconiapowders were obtained from NanoProducts Corporation (Longmont, Colo.80504, USA). The physical characteristics of these powders are listed inTable 3. Also listed are the properties of TZ-3YS-E powder availablefrom Tosoh Corporation (Tokyo, Japan), a conventional so-called “easysintering” 3 mol % yttria-stabilized tetragonal zirconia powder that wasused for baseline comparison. The nano-powders were mixed with 5-10 wt.% PVA binder (Elvanol 50-42, Dupont) with a mortar and pestle, and thensieved through an 80 mesh screen resulting in free-flowing, pressablepowder. The conventional zirconia powder was pressable to begin with, asit contained binder.

The powders were pressed into pellets using a double action die and thenvacuum bagged and cold isostatically pressed (CIPed) at 400 MPa toremove any green density gradients. The resulting pellets wereapproximately 3 grams in weight and measured about 12 mm in diameter andabout 7 mm in height. The green density of conventional and nanosizedzirconia pellets were approximately the same, 54±0.2% and 52.6±1.2%,respectively. The pellets were then burned out by heating at a rate of2° C./m to 700° C. and holding for 2 hours. The pellets weresubsequently sintered at 1200-1300° C. for 2 hours with a heating andcooling rate of 4° C./m. Density of the sintered pellets was measured bythe Archimedes method using water as the immersion medium. The percenttheoretical density was calculated using a theoretical density value of6.05 g/cm³. These results are shown in FIG. 3 and demonstrate that attemperatures below 1250° C., the nano-sized zirconia samples exhibitenhanced sintering behavior at lower temperatures versus theconventional zirconia sample. For example at 1225° C. the NP-2 pelletsdensified to 94.1±0.2% which compares to 86.5±0.2% for the conventionalzirconia sample. This improved sintering behavior is attributed to thesmaller crystallite and particle size of the NP-1 powder (see Table 3).This sintering enhancement due to smaller crystallites/particles is alsoreflected by the data at 1200° C., which shows a progressive increase indensity from the conventional zirconia sample (74.5±0.3%) to thenanparticulate zirconia samples, NP-1 (83.1±0.1%) and NP-2 (85.8±0.1%).To reduce sintering temperatures below 1000° C. the average particlesize should be reduced to below about 8 microns as demonstrated by Kimeland Skandan et al. (cited above).

TABLE 3 Crystallite Particle Size Specific Surface Powder Size (nm) (nm)Area (m²/g) NP-1; NanoProducts Corp. 10.2 17.2 59.6 (Longmont, CO, USA)product number ZR3N3063 NP-2; NanoProducts Corp. 8.8 15.3 67.1(Longmont, CO, USA) product number ZR3N3269 Conventional 3Y-Zirconia; 3690 7 ± 2 Tosoh product TZ-3YS-E

Example 3 further illustrates that some of these nanopowders can besintered isotropically using simplified cup shape geometry.

EXAMPLE 3

Using the methods described in example 1, green cylindrical shapedbodies with dimensions of ˜12 mm diameter by ˜12 mm height were formedout of the NP-2 and conventional zirconia powders. A 5 mm diameter×8 mmdeep hole was machined into the green bodies yielding a “cup” geometry.This geometry was chosen since it simulated a dental coping. Afterrecording the orthogonal dimensions, L_((x,y,z)) where x and y refer tothe diameters taken at a 90° rotation to each other and z refers to theheight as illustrated in FIG. 4, the green bodies were burned out byheating at a rate of 2° C./m to 700° C. and holding for 2 hours, andsubsequently sintering at 1225° C. for 2 h with a heating and coolingrate of 4° C./m. The dimensions of the sintered bodies were recorded andthe sintering shrinkage was calculated. These results are shown in FIGS.5 and 6. These data show that greater shrinkages were achieved for thenanosized zirconia powder versus the conventional zirconia, which agreeswith the sintering results shown in Example 2. Additionally, asmanifested by the data points falling on the line of isotropic shrinkagerepresented by the dashed lines, these results demonstrate that like theconventional zirconia the nanosized zirconia also densifies and shrinksisotropically during sintering.

EXAMPLE 4

Commercially available nano-sized alumina powder AL3N3197 obtained fromNanoProducts Corporation (Longmont, Colo. 80504, USA) was premixed intoNP-2 zirconia nano-powder used in Examples 2 and 3 above. The physicalcharacteristics of alumina nano-powder are compared to NP-2 below:

TABLE 4 Powders from NanoProducts Corp. Crystallite Particle Specific(Longmont, CO, USA) Composition Size (nm) Size (nm) Surface Area (m²/g)NP-3 (product # AL3N3197), 99.9% Al₂O₃ 4.3 8.6 175.2 Al₂O₃ nanopowderNP-2 (product # ZR3N3269) 94.7% ZrO₂ + 8.8 15.3 67.1 YTZP nanopowder5.3% Y₂O₃

The blend of 0.5 wt % of NP-3 and 99.5 wt % of NP-2 nano-powders wasmixed with 5-10 wt % PVA binder (Elvanol 50-42, Dupont) with a mortarand pestle, and then sieved through an 80 mesh screen resulting infree-flowing, pressable powder. The powder was vacuum bagged and coldisostatically pressed at 400 MPa to produce green billets of about 1-2inches in diameter and about 5-10 inches in length. Following the coldisostatically pressing step, the outer layer of the billets were removedby turning to eliminate any green density gradients that may haveexisted in the outer layer. The billet was further sectioned intoshorter cylinders of about 30 mm in diameter and about 50-60 mm inheight. The cylinders were then debinderized and pre-sintered to about50% theoretical density in a two step firing cycle comprising heating atthe rate of 1° C./minute to about 700° C. and holding for about 2 hoursat this temperature followed by a 2 hour hold at about 900° C. Theattained bisque densities and the anticipated Bisque-to-Final linearshrinkages were calculated for each individual block based on diameterand height measurements before and after pre-sintering.

The pre-sintered cylinders are subsequently used to mill the enlargedframeworks for dental restorations. Each framework is enlarged based onthe linear shrinkage factor calculated for each individual pre-sinteredcylinder from which the framework is milled. The milled frameworks aresintered at 1250° C. for 4 hours with a heating rate of 2° C./minute todensities exceeding about 95% theoretical density as determined by theArchimedes method. The isotropic shrinkage in the frameworks isconfirmed by fitting frameworks on the original master model. Some ofthe frameworks were layered by 3G Porcelain (Pentron® LaboratoryTechnologies, LLC, Wallingford, Conn.) to demonstrate the finishingsteps typical in fabrication of aesthetic all-ceramic dentalrestorations.

The examples above demonstrate that the selected nanosized powdersexhibiting sinterability below 1300° C. were consolidated into shapesthat were sintered isotropically, i.e. without distortion whereinL_(x)=L_(y)=L_(z)=L_(Diameter)=L_(Height)where L is the linear shrinkage. It was observed that sintering of thecompacts of the nanopowders capable of isotropic sintering results innearly fully densified articles with the average grain size noticeablylarger than 100 nm. Two major difficulties in processing dental articlesusing nanopowders were revealed: (1) the consolidation of bulk shapes byconventional methods such as powder compaction and slip-casting; and (2)sintering to achieve densities in excess of 95%.

To overcome the first processing obstacle mentioned above, solid freeform manufacturing methods such as rapid prototyping or solid imagingare utilized indirectly in combination with other processing techniquessuch as injection molding/heat-pressing, various coating or depositiontechniques such as gel casting, slip casting, slurry casting, pressureinfiltration, dipping, colloidal spray deposition, direct coagulation asdescribed in U.S. Pat. Nos. 5,667,548, 5,788,891 and 5,948,335, whichare hereby incorporated by reference, and electroforming orelectrophoretic deposition techniques. While SFF methods are used tofabricate enlarged substrates, dies and molds, any of the above listedtechniques can be used to form nanoparticulate materials of theseinvention into green shapes conforming to these substrates or molds. Theelectroforming is a preferred method since it utilizes suspensions whichare particularly beneficial for the nanomaterials herein described. Manyof the nanoparticulate materials described herein are more readilyobtained as well-dispersed suspensions rather than free-flowing powders.Example 5 illustrates electroforming as the preferred method ofdepositing ceramic nanoparticulates onto enlarged dies produced by oneof solid free form manufacturing methods. Yet another preferredtechnique is low-pressure injection molding into negative molds of therapid prototyped models, or alternatively existing heat pressingequipment can be used for pressing into refractory investment moldsproduced by lost wax technique. Example 6 illustrates low-pressureinjection molding as another preferred method of forming ceramicnanoparticulates using enlarged molds produced by one of SFF methods.

To alleviate the second processing obstacle mentioned above, thenanopowders herein are agglomerated with sintering aids such as Si, Al,Mg, Zr, Y, Ce, Ta and mixtures thereof, and grain growth inhibitors suchas Cr, Ti, Ni, Mn and mixtures thereof. Depending on the subsequentprocessing steps, these additions can be added in their elemental(metallic) form or in the form of oxides, salts, organometalliccompounds, or other precursor compounds, in the form of colloids,powders and specifically nanopowders. To further lower the meltingtemperature, the inclusion of the above-mentioned additives asnanopowders or precursor compounds, is most preferred.

EXAMPLE 5

An optical scanner, ZFN D-21, available from ZFN (ZahntechnischesFraszentrum Nord GmbH & Co. KG, Warin, Germany) is utilized to scanmaster models (dies) made from impressions comprising preparations forbridges and crowns. Three-dimensional CAD software provided with a ZFND-21 scanner is used to design frameworks and copings corresponding tothese master models (dies). 3D CAD files (solid models) of theseframeworks and copings are enlarged using the linear shrinkagecoefficient corresponding to the anticipated sintering shrinkage of thenanozirconia materials of the present invention, saved asstereolithography (.STL) files and transferred to a computer interfacedwith an RP (Rapid Prototyping) machine such as Perfactory® Miniavailable from Envision Technologies GmbH (Marl, Germany). This machineutilizes a photostereolithography process also known as digital lightprocessing (DLP) to build three-dimensional objects from a light curableresin. Fifteen units are built at the same time layer by layer with anindividual layer thickness of about 50 microns. Individual units areseparated, attached to copper wire electrodes and coated with conductivesilver paint (silver lacquer) available from Gramm GmbH or WielandDental+ Technik GmbH & Co., KG (Pforzheim/Germany). AN electroformingunit, such as AGC® Micro Plus (Wieland Dental+ Technik GmbH & Co. KG),is used to deposit a dense layer of yttria-stabilized zirconiapolycrystals (YTZP) from an ethanol based suspension as described inExample 3 of U.S. Pat. No. 6,059,949, which is hereby incorporated byreference. An electroforming suspension is prepared by suspending NP-2zirconia powder available from NanoProducts Corp. (Longmont, Colo.) inpure ethanol with addition of 0.05% vol. acetyl acetone dispersant and0.1% vol. of 5% wt. PVB (polyvinyl butyral binder) in pure ethanol.Alternatively, an ethanol-based suspension is prepared from an aqueoussuspension comprising tetragonal nano-zirconia particles of about 8 nmaverage particle size. First, aqueous suspensions of YTZP having acrystal size of about 8 nm are prepared via precipitation fromhomogeneous solutions using complexation chemistry techniques. Zirconiumand yttrium salts, ZrO(NO₃)₂·xH₂O (zirconyl nitrate, Aldrich Chem.,Milwaukee, Wis.) and Y(NO₃)₂·6H₂O (yttrium nitrate hexahydrate), AldrichChem., Milwaukee, Wis.) are each dissolved in CO₂-free deionized waterin the appropriate amounts to achieve 0.5 M solutions of each. These arethen mixed, in the appropriate ratio to yield the desired mol. % of Y₂O₃in the final powder, with the complexing agent bicine(www.sigmaaldrich.com) (2:1 bicine:Zr (mol)). The pH of this feedsolution is adjusted to about 13 by additions of TEAOH(Tetraethylammonium Hydroxide, Aldrich Chem., Milwaukee, Wis.), and thesolution is then put in a teflon-lined hydrothermal vessel (ParrInstrument Company, Moline, Ill.), which is heated to 200° C. for 8hours to hydrothermally synthesize YTZP crystals of about 8 nm in size.

Aqueous suspensions are converted into alcohol-based suspensions bycentrifuging and then redispersing in ethanol. The average thickness ofthe electrophoretic coating is about 0.5-0.6 mm. Followingelectroforming of the powders onto the substrates, sintering is carriedout in a Deltech furnace using a two-step firing cycle comprisingheating rate of 1° C./min to about 450° C., holding at this temperaturefor 2 hours to remove organics, further heating at a rate of 1° C./min.to 900° C.-1100° C. and holding at this temperature for about 2 hours.Densities in excess of 90% of theoretical density can be achieved.

EXAMPLE 6 Low-Pressure Injection Molding (LPIM) with Peltsman Unit

An optical scanner, ZFN D-21, available from ZFN (ZahntechnischesFraszentrum Nord GmbH & Co. KG (Warin, Germany) is utilized to scanmaster models (dies) made from impressions comprising preparations forbridges and crowns. 3D CAD software provided with a ZFN D-21 scanner isused to design frameworks and copings corresponding to these mastermodels (dies). 3D CAD files (solid models) of these frameworks andcopings are enlarged using the linear shrinkage coefficientcorresponding to the anticipated sintering shrinkage of the nanozirconiamaterials of the present invention, saved as stereolithography (.STL)files and transferred to a computer interfaced with an RP (RapidPrototyping) machine such as Perfactory® Mini available from EnvisionTechnologies GmbH (Marl, Germany). This machine utilizesphotostereolithography process also known as digital light processing(DLP) to build 3D objects from a light curable resin. Fifteen units arebuilt at the same time layer by layer with an individual layer thicknessof about 50 microns. Individual units are separated and molded in aliquid silicone rubber (Silastic® M RTV Silicone Rubber from Dow CorningCorporation) which is castable and easily demolded after curing toproduce negative molds for low-pressure injection molding. It should benoted that instead of using silicone negative molds, the molds for LPIMcan be designed and fabricated directly using the Perfactory® Mini RPmachine and the supplied software.

Feedstock containing nanosized zirconia for injection molding isprepared from a binder comprised of 75 wt % of paraffin wax (meltingpoint of 49°-52° C.), 10 wt % of polyethylene wax (melting point of80°-90° C.), 10% of carnauba wax (melting point of 80°-87° C.), 2 wt %of stearic acid (melting point of 75° C.) and 3 wt % of oleic acid(melting point of 16° C.) readily available from a number of suppliers.Nanosized zirconia having a crystallite size of about 19 nm and particlesize of about 15 nm (available as Product Number ZR3N3269 fromNanoProducts Corp., Longmont, Colo. 80504, USA) is used. The mixing isdone directly in a low pressure molding (LPM) machine, (Model MIGL-33available from Peltsman Corporation, Minneapolis, Minn.) at atemperature of 90° C. The feedstock mixture is comprised of about fiftypercent (50%) by volume of a binder. Once the feed stock mixture isthoroughly mixed it is injected into the cavity of the silicone rubbermolds at a pressure of approximately 0.4 MPa and a temperature ofapproximately 90° C. The injection-molded green part is then demoldedfrom the silicone mold, which is done easily due to elasticity of thesilicone. Green densities of approximately 50% were achieved. The greenbodies were debinderized and sintered to nearly full density asdescribed above.

EXAMPLE 7 Injection Molding with Autopress

Feedstock containing nanosized zirconia for injection molding isprepared from a binder comprised of paraffin wax, with minor proportionsof polyethylene wax, carnauba wax, stearic and oleic acids using thesame formulation as used in Example 6. Nanosized zirconia having acrystallite size of about 19 nm and a particle size of about 15 nm(available as Product Number ZR3N3269 from NanoProducts Corp., Longmont,Colo. 80504, USA) is used. The mixing is done in a KitchenAidProfessional 5 mixer (St. Joseph, Mich.) in a bowl continuously heatedto 90° C., which is above the melting point of the binder. Heating isachieved using a high temperature heat tape available from McMaster-Carr(New Brunswick, N.J.). The heat tape is wrapped around the mixer bowl toprovide heat to the bowl. After cooling to room temperature, theresulting mix is crushed into powder to a 60 mesh (250 μm) particle sizeusing a mortar and pestle. This powder is then ready for injection intothe cavity of a mold. Additionally, the mix can be cast into pellets bypouring into a metal “clam-shell” mold, while still in the molten state.

Previously acquired stereolithography (*.STL) files of bridge frameworksand crown copings were sent to microTEC, Bismarckstrasse 142 b 47057Duisburg, Germany) for production of the enlarged replicas usingRMPD®-mask technology via toll rapid prototyping service availablethrough microTEC's website. The replicas were fabricated in a layerthickness of twenty five microns (25 μm) from photo-curable resin.

The resulting replicas are invested in Universal™ Refractory Investment(available from Pentron® Laboratory Technologies, LLC, Wallingford,Conn.). After the investment has hardened, the resin replicas inside areeliminated by placing it into a preheated furnace thereby burning offthe resin, resulting in a mold cavity for forming the dental article.The injection molding feedstock, in free-flowing granule or pellet form,as described above, is then placed into the investment mold assembly,which is then transferred into the pressing unit. It is pressed into theinvestment ring using an AutoPressPlus® (Pentron® LaboratoryTechnologies, LLC, Wallingford, Conn.). having an external aluminaplunger Pressing is done at approximately 90° C., and after cooling thepressed green part is then carefully divested by sand-blasting withglass beads at a pressure of 15 psi and the plunger and mold aredisposed of. Green densities of approximately 50% were achieved. Thegreen bodies were debinderized and sintered to nearly full density asdescribed above.

It should be noted that in all the cases described in Examples 1-7 itwas observed that while the ceramic portion of the starting powder,suspension or feedstock consists of crystallites with average sizes ofless than 20 nm, the sintered dental articles have average grain sizeswithin the range from about 100 nm to about 450 nm. It is the nature ofthe materials of the present invention to exhibit substantial coarseningconcurrent with densification wherein the final grain size is about10-20 times larger than the starting crystallite size.

Though not within the scope of the present invention which is directedtowards sintering ceramic dental articles comprising nanopowders tonearly full density, nevertheless, it should be noted that the injectionmolding technology described in Examples 6 and 7 can be used to producedental articles even if access to RP machines is not available. In thelatter case, if is not possible to make enlarged replicas and greenbodies fabricated therefrom as described in Examples 6 and 7, thearticles will have to be presintered without shrinkage and glassinfiltrated as described in U.S. Pat. Nos. 4,772,436 and 5,910,273,which are hereby incorporated by reference. In the case of YTZP zirconiacores, 3G porcelain (Pentron® Laboratory Technologies, LLC, Wallingford,Conn.) can be used for both glass infiltration and esthetic layering ofthe resulting glass-infiltrated cores.

While various descriptions of the present invention are described above,it should be understood that the various features can be used singly orin any combination thereof. Therefore, this invention is not to belimited to only the specifically preferred embodiments depicted herein.

Further, it should be understood that variations and modificationswithin the spirit and scope of the invention may occur to those skilledin the art to which the invention pertains. Accordingly, all expedientmodifications readily attainable by one versed in the art from thedisclosure set forth herein that are within the scope and spirit of thepresent invention are to be included as further embodiments of thepresent invention. The scope of the present invention is accordinglydefined as set forth in the appended claims.

1. A dental article manufactured by the process comprising: mixingceramic powder comprising nanoparticles having a crystallite size ofless than about 20 nm with a sintering aid comprising metallicparticles, wherein the metallic particles have a crystallite sizegreater than about 100 nanometers; forming the powder mixture into adental article; subjecting the article to microwave radiation to densifythe article; wherein the final ceramic material exhibits at least 30%relative transmission of visible light when measured through a thicknessin the range of about 0.3 to about 0.5 mm.
 2. The dental article ofclaim 1 further comprising mixing a binder with the ceramic powder andsintering aid.
 3. The dental article of claim 1 wherein the ceramicpowder comprises nearly spherical particles.
 4. The dental article ofclaim 1 wherein the dental article is selected from the group consistingof orthodontic retainers, bridges, space maintainers, tooth replacementappliances, splints, crowns, partial crowns, dentures, posts, teeth,jackets, inlays, onlays, facings, veneers, facets, implants, cylinders,abutments and connectors.
 5. The dental article of claim 1 wherein theceramic powder comprises zirconia, alumina, titania, silica, magnesia,yttria, ceria and mixtures thereof.
 6. The dental article of claim 5wherein the zirconia is yttria-stabilized tetragonal zirconia.
 7. Thedental article of claim 2 wherein the binder is selected from the groupconsisting of polyacrylic binders, polyvinyl alcohol (PVA) binders,polyvinyl butyral (PVB) binders, waxes, and mixtures thereof.
 8. Thedental article of claim 7 wherein the waxes comprise natural waxes,synthetic waxes or mixtures thereof.
 9. The dental article of claim 8wherein the waxes comprises paraffin wax, polyethylene wax, carnaubawax, bee wax, stearic and oleic acids, and mixtures thereof.
 10. Thedental article of claim 1 wherein the metallic particles are added asoxides, salts, organometallic compounds, or precursor compounds.
 11. Thedental article of claim 1 wherein the metallic particles are added inthe form of colloids or powders.
 12. The dental article of claim 1wherein the metallic particles are selected from the group consisting ofSi, Al, Mg, Zr, Y, Ce, Ta metal and mixtures thereof.
 13. The dentalarticle of claim 1 wherein forming the powder mixture into a dentalarticle is performed by hand, by CAD/CAM methods or by rapid prototypingmethods.
 14. The dental article of claim 1 wherein the dental article ispolycrystalline and comprises grains greater than about one hundrednanometers.
 15. The dental article of claim 1 wherein the dental articlecomprises grain sizes from about one hundred nanometers to about 500nanometers.
 16. The dental article of claim 1 wherein the metallicparticles are microscopic in size.
 17. The dental article of claim 1wherein the metallic particles are macroscopic in size.
 18. The dentalarticle of claim 14 wherein the polycrystalline grains are comprised ofclusters of nanocrystallites.
 19. The dental article of claim 14 whereinthe polycrystalline grains comprise nano, micro, and macro particles.